This invention relates to a phase shift mask for use in exposing and transferring a fine pattern and a phase shift mask blank as a base material thereof, and in particular, to a half-tone type phase shift mask and a phase shift blank thereof.
Recently, high integration of a DRAM (Dynamic Random Access Memory) has been transferred from 1 Mbit into 64 Mbit or 256 Mbit. In this event, mass-production of the DRAM of the 64 Mbit or the 256 Mbit has been established in the recent years. With such a technology advancement of the DRAM, an exposure optical source has been shortened in wavelength from g line (436 nm) into i line (365 nm) in the known mercury lump of super high pressure.
Now, development has been made about shortness of exposure wavelength to achieve further high integration.
Herein, although the shortness of the exposure wavelength serves to improve resolution, focal depth is inevitably reduced. This increases design load of an exposure optical system, remarkably reduces process stability, and further gives an adverse effect for manufacturing yield.
The known phase shift method is one of methods of transferring a super resolution pattern for effectively resolving the above-mentioned problems. In such a phase shift method, a phase shift mask is used as a mask for transferring the fine pattern.
For instance, the phase shift mask is composed of phase shifter portions which form pattern portions on a mask and non-pattern portions (namely, substrate exposing portions) which have no phase shifters.
With such a structure, phases of light beams which transmit through the phase shifter portions and the non-pattern portions are deviated with about 180.degree. to each other. Thereby, the light beams are mutually interfered at border portions of every patterns. In consequence, contrast of the transferred image is enhanced.
Further, it is possible to increase the focal depth to obtain the required resolution by the use of the phase shift method. Moreover, both resolution and process applicability can be enhanced by using the light beam having the same wavelength, as compared to the transferring process using the normal mask having a general shielding pattern of a chromium film.
In the meanwhile, the phase shift mask is practically and generally classified into the perfect-transmit type phase shift mask (namely, Shibuya, Lebenson type) and the half-tone type phase shift mask. The former has optical transmittance equivalent to the non-pattern portion (namely, the substrate-exposing portion), and is an approximately transparent mask for exposure wavelength, and is generally effective to transfer the line and space.
On the other hand, the latter has the optical transmittance of about several % to several tens % for the non-pattern portion (the substrate exposing portion) in the phase shifter portion, and is effective for fabricating a contact hole or an isolated pattern in a semiconductor manufacturing process.
In this case, basic structures of the half-tone type phase shift mask blank and the half-tone type phase shift mask are illustrated in FIGS. 1 and 2, respectively. Herein, the half-tone type phase shift mask may be thereinafter abbreviated as a phase shift mask while the half-tone type phase shift mask blank may be thereinafter abbreviated as a phase shift mask blank.
In this event, a reflection-preventing layer and an etching stop layer, which may be used in the lithography process, are omitted.
The phase shift mask blank is formed by depositing a semi-transparent film 2 (namely, a half-tone phase shifter film) on a transparent substrate 1, as illustrated in FIG. 1.
On the other hand, the phase shift mask is composed of the phase shifter portions 3 which form the pattern portions on the mask and the non-pattern portions 4 (the substrate exposing portions) which have no phase shifter, as illustrated in FIG. 2.
In this condition, the phase shifter portion 3 serves as a phase shifter by shifting phase of an exposure light beam which transmits a portion around the edge thereof, and further serves to substantially shield the exposure light beam for a resist on a substrate to be transferred. In this case, the phase shift mask is a single layer type. Herein, the single layer type is simple in structure, and further easy in fabrication.
Such a single layer type is a phase shifter consisting of a chromium based material, such as, CrO.sub.X, CrN, CrO.sub.X N.sub.Y, and Cr.sub.X ON.sub.y C.sub.2, which is disclosed in Japanese Unexamined Patent Publication No. H5-127361.
Further, the single layer type may be a phase sifter consisting of a MoSi based material, such as, MoSiO and MoSiON, which is disclosed in Japanese Unexamined Patent Publication No. H6-332152.
Moreover, the single layer type may be a phase sifter consisting of a SiN based material or a SiO based material, which is disclosed in Japanese Unexamined Patent Publication No. H7-261370.
In recent years, the phase shift mask has been voluntarily used with the shortness of the exposure wavelength. Recently, the krypton fluoride (KrF) excimer laser light beam (248 nm) is often used as a light beam which has wavelength shorter than the i line.
Further, suggestion has been made about using the argon fluoride (ArF) excimer laser light beam (193 nm) or the argon chloride (ArCl) excimer laser light beam (175 nm).
With such shortness of the exposure wavelength, it is important to control optical factors, such as transmittance and refractive index of the exposure wavelength to be used in the corresponding phase shift mask and phase shift mask blank.
It is remarkably difficult to control the desired transmittance in shorter region in wavelength than 250 nm with respect to many substances. This is because absorption degree of the light beam remarkably becomes large different from a region between visible region and near violet region. Therefore, the phase shift mask itself for the i line can not be generally used as the phase shift mask for the exposure light beam having wavelength shorter than 250 nm.
Under this circumstance, the setting of the transmittance of the phase shifter is determined by sensitivity of the resist used in a pattern transferring process and a patterning process.
For instance, it is desirable that the transmittance of the exposure light beam can be controlled within the range between 3% and 20% in a film thickness of the phase shifter which shifts the phase of the exposure light beam to the reselected angle in case of the phase shift mask.
Even when the above-mentioned basic requirement characteristics, such as the transmittance and the refractive index for the exposure wavelength are satisfied in accordance with the shortness of the exposure wavelength, inspection can not be carried out when the transmittance for the wavelength (for example, 364 nm, 488 nm, and 633 nm) of an inspection light beam is high.
Consequently, it is not suitable for the practical use. Therefore, it is required to control the transmittance for the wavelength of the inspection light beam to the desired value for the practical use.
Further, the following characteristics are required in addition to the above-mentioned characteristics for the phase shift mask and the phase shift mask blank.
Namely, it is necessary that they are stable for irradiating the used excimer laser light beam (resistance to light beam), that they have chemical durability for a washing process (resistance to chemical agent) requisite to the mask process, and that fine defects in the blank, which remarkably deteriorate quality of the mask, are minimized (low defect density).
In more detail, the shortness of the exposure wavelength means that the energy density which is irradiated per a unit time is increased at the same time. In accordance with this, it is required that function of the phase shift mask is not degraded by the damage due to the light irradiation in the film material for forming a phase shifter film.
Herein, it is to be noted that the damage means change of the optical characteristics (the refractive index and the transmittance) of the phase shifter film, generation of a color defect, change of a film thickness, and deterioration of film quality, which are caused by the light beam irradiation.
For instance, it has been said that when the excimer laser, which has wavelength in a deep ultraviolet region, is irradiated, substance in the film is exited by the two photons process. This results in the above change of the optical characteristics of the film and the film quality. However, the detail process has not been clarified yet.
At any rate, it is one of essential conditions that the phase shifter has high resistance to irradiation in the irradiation of high energy light beam accompanying with the shortness of the exposure wavelength.
Further, when material of the shifter film is considered from the viewpoint of the mask material, the film should not be deformed or dissolved by the washing due to acid or alkai in the mask manufacturing process. That is, the chemical durability is required for the phase shifter film in spite of length of the exposure wavelength.
Moreover, when the phase shift mask is considered from the viewpoint of a tool for performing a fine process, the fine process characteristic is required to achieve the process (patterning or etching) of the phase shift mask blank at high accuracy. To this end, it is necessary that the phase shifter film is uniform and further, has no defect.
In the meanwhile, it is said that miniaturization of the mask pattern proceeds with the shortness of the exposing wavelength in future. Therefore, reliability of the pattern transferring largely depends upon the defect in the phase shifter film. Thus, the defect in the phase shifter film will become an important problem.
However, the conventional phase shift mask and the blank thereof do not sufficiently satisfy the basic requirement characteristics, such as, the transmittance accompanying with the shortness of the exposure wavelength and the index of the refraction and the above-mentioned other requirement characteristics.